The invention relates to an alarm system, and in particular to an alarm system designed to protect an enclosed spaced and give warning that the space has been penetrated by an intruder. The space may be a domestic or commercial building, a room in such a building, a safe, a vault or the interior of a vehicle.
In U.S. Pat. No. 3,947,838 there is described an alarm system comprising a sensor responsive to air pressure within an enclosed space, the sensor providing electrical signals related to the sensed air pressure, and a signal processor to which the electrical signals are supplied and operative to initiate an alarm indication when the signal supplied by the sensor is indicative of an intrusion into the enclosed space.
In this known system the signal processor is operative to distinguish between signals supplied by the sensor indicative of an intrusion into the enclosed space and such signals deriving from changes in the air pressure in the enclosed space caused by changes in the ambient atmospheric conditions in and around the enclosed space. The signal processor effects such distinction between the different signals it receives in dependence upon the rate of change of the value of the signal, on the basis that intruder entry into the enclosed space cannot be accomplished in less than a minimum predetermined time and will be completed in less than a maximum predetermined time.
A disadvantage of this known system is that it is not completely compensated for changes in ambient atmospheric conditions and may therefore give a false alarm if such conditions change in an unexpected manner and give a rate of change of air pressure in the enclosed space similar to that produced by an intruder.